Infrared image converter

ABSTRACT

Device for converting light in the infrared portion of the spectrum into visible light based on the phenomenon of thermal quenching of visible fluorescence comprising a conversion screen which contains an ultraviolet irradiated temperature-sensitive phosphor.

I United States Patent [15] 3,639,765

Kleinerman 1 Feb. 1, 1972 [54] INFRARED IMAGE CONVERTER 2,527,36510/1950 Leverenz ..252/301.4 S [72] Inventor: Mums Kkinermm Pain Breeze2,563,472 8/1951 Leverenz ..250/83.3 HP Webster, Mass. 01570 OTHER pU OFiled: y 1, 1969 Luminescence of Inorganic Solids, Academic Press,Risdone 211 App]. No.: 838,088

Primary Examiner-James W. Lawrence [52] US. Cl. ..Z50/83.3 H, 250/7] R,252/3012 R Assistant Examiner-D. C. Nelms [51] Int. CL... 60 1/16Attorney-Lane, Aitken, Dunner & Ziems [58] Field olSearch..250/83.3,7l,80,213 11,213 VT;

313/108 R, 108 C, 108 D; 252/3012 R [57] ABSTRACT Device for convertinglight in the infrared portion of the spec- [56] Re'ems Cited trum intovisible light based on the phenomenon of thermal UNITED STATES PATENTSquenching of visible fluorescence comprising a conversion screen whichcontains an ultraviolet irradiated temperature- 2,482,815 9/1949 Urbach..250/83.3 Sensitive phosphor 2,989,643 6/1961 Scanlon...

Borst ..250/83.3

19 Claims, 4 Drawing Figures PATENIEDFEB H972 INTENSITY sum 1 or 2 -x LL4 f E A, 2

INVENTDR I I I l MARCOS KLEINERMAN WAVELENGTH-NANOMETERS %MMWW A ORNEYSPATEMTEum 1m 3339.765

SHEET E'UF 2 mvm MARCOS KLEINERM AT RNEYS INFRARED IMAGE-CONVERTERBACKGROUND OF THE INVENTION The field of the invention is infrared imageconverters. Infrared image converters based on the phenomenon of thermalquenching of visible fluorescence excited by ultraviolet radiation isknown and a discussion of the principles involved can be found in anarticle by F. Urbach, N. R. Nail and D. Pearlman, Journal of OpticalSociety of America, Vol. 39, p. 1,01 l (1949), the teachings of whichare expressly incorporated herein by reference. This method of imageconversion is known as fluorescence thermography." Prior to the presentinvention, the most sensitive phosphors known and used in fluorescencethermography" had a temperature coefiicient of fluorescence intensity of25 percent per degree centigrade at near room temperatures and becauseof the relatively large heat capacity of solids in this temperatureregion fluorescence thermography is not sensitive enough to besuccessfully used in an infrared projection thermography system. In sucha system an infrared image is made visible by projecting the signal ontoan ultraviolet irradiated temperature sensitive phosphor which forms animage from the energy .of the signal.

The disadvantages of fluorescence therrnography" for infrared systems isovercome in accordance with this invention by a conversion screen oflow-temperature phosphors which by thermal quenching or thermalenhancement improve the sensitivity over the prior art by about four ormore orders of magnitude.

SUMMARY OF THE INVENTION A fluorescent screen is provided of aluminescent system in which the system has two kinds of emissive centerswhich can be identified as A centers and B centers, which fluoresce attwo different wavelengths A, and A and with a high total fluorescenceyield (greater than 0.1) when excited with long wavelength ultravioletradiation. The system is chosen so that the concentration of the Bcenters is much smaller than that of the Acenters. Thus energy transfercannot occur efficiently from an A center to a B center except via anenergy level which is higher than that ofthe A center.

By providing such a screen with characteristics as described below asensitivity can be attained which is sufficient to enable the conversionof infrared light to visible light.

Accordingly, it is an object of the invention to providean improveddevice for converting infrared energy to visible light.

BRIEF DESCRIPTION OF THEDRAWING DESCRIPTION OF THE PREFERRED EMBODIMENTSIn the present invention, a fluorescent screen is made of a luminescentsystem as shown in FIG. 1, in which the system has two kinds of emissivecenters which can be identified as A centers and B centers, which have ahigh total fluorescence at two different wavelengths A, and A and with ahigh total fluorescence yield 0.1 when excited with long wavelengthultraviolet radiation. The system is chosen so that the concentration ofthe B centers is much smaller than that of the A centers. Thus energytransfer cannot occur efficiently from an A center to a B center exceptvia an energy level X of the medium as is shown diagrammatically inFIG. 1. Level X has an energy level higher than that of the A centers bya value vAE.

Under ultraviolet irradiation the A center levels are populated, andsome of the energy migrates to the B centers which will fluoresce withan intensity I,,. The rate K of energy transfer from A center to Bcenter is given by (1) K =C exp (-AE/kT) and is discussed in an articleby M. Kleinerman and S. Choi. J. Chem. Phys, Vol. 49, p. 3,901 1968),the teachings of which are herein incorporated by reference.

In the above equation, C is a constant for a given sample, which dependson the concentration of B centers and that of other exciton traps, k isthe Boltzman constant and T is the temperature in degrees Kelvin. At twodifferent temperatures T, and T the ratio of energy transferefficiencies in the absence of other phenomena is:

If T is the temperature of the screen in the absence of incidentinfrared radiation, and if AE is equal to approximately 10k T, afraction of the energy from the excited A centers will be transferred tothe B centers. When infrared radiation is absorbed in the screen thetemperature rise at the point of incidence will increase the rate ofenergy transfer, and hence I, in accordance with eq. (.2). For smalltemperature changes the condition AE=1 OkT leads to:

(3) AT=T/-1O 1n Tim/I, At T=4.2 K. eq. (3) becomes:

I log lag/In The amount of heat Q absorbed from the infrared radiationneeded to produce the increase AT in a gram mole of the phosphor is:

Q=0-96 C" l g, na/ m where C,, is the molar specific heat of thephosphor.

Since C,, is about three orders of magnitude smaller at T=4.2 K. than at300 K., it follows from eqs. (3) and (5) that the energy required toproduce the needed ratio 1 /1, is about five orders of magnitude smallerat liquid helium temperatures than at 300 K.

Systems somewhat different from those described in FIG. I, but alsoinvolving activated exciton migration, are also contemplated forinfrared image conversion. The requirement is only that the systems havesimilar sensitivities. The following variations are possible:

Variation 1. A centers luminescent; B centers nonluminescent.

Variation II. A centers nonluminescent; B centers luminescent. 7

Variation 1 offers the possibility of a significant thermalampliflcation effect. Thus, the quenching of the fluorescence of the Acenters caused by the temperature rise AT will produce an additionaltemperature rise A,T from the conversion of the fluorescence into heat.

Crystalline Tb chelates doped with Eu have been found useful inpracticing the invention since with Eu doped Tb chelates, the tripletlevel of the ligand (the X level of FIG. 1) can be slightly above the Dlevel of Tb (the A center) so that the excitation energy can migrate tothe europium ions (the B centers).

The behavior of one such system is illustrated in FIG. 2 where thefluorescence intensity versus wavelength in nanometers is plotted. FIG.2 shows the total fluorescence from a phosphor having both centers A andB. The fluorescence from the Eu ion at 18 K. and 20 K. is indicated byreference numerals 2 and 3 respectively while the fluorescence from theTb ion at 18 K. and 20 K. is indicated by reference numerals 4 and 5respectively. The phosphor selected is the crystalline chelate terbiumtetrakis [1,3 bis(p-methoxyphenyl) 1,3-propanedione] piperidine dopedwith 1 percent of the corresponding europium chelate. As is shown inFIG. 2, a AT of 2 K. has greatly reduced the fluorescence from Tb (the Acenters) while increasing the fluorescence from Eu (the B centers).

Exciton transport and fluorescent emission in chelates of the typedescribed takes place in a total time of about 10 seconds or less, ifthe Eu dopant concentration is greater than I percent. This speed issufficient for most purposes because the time for electronic scanning ofthe image is usually longer. The time resolution will therefore belimited by the thermal relaxation time of the fluorescent screen, andthis can be controlled by the extent of thermal coupling of the screento the cold finger of the cyrogenic apparatus, which is described below.

It is preferred that systems with a high total fluorescence quantumefficiency and a value of AE of about 25 to 35 Kaysers be employed.Terbium chelates doped with europium or other exciton traps meet thisrequirement because the position of the emissive D, level of Tb ispractically invariant upon combination of Tb with different ligands.However, the ligand which combines with Tb to form a crystalline chelatemust end up with a triplet level about 25 to 35 Kaysers above the Dlevel of Tb. The value of AE in the crystalline chelate can bedetermined from eq. (7) (given below). The ligands useful for practicingthe invention are those with a triplet level T within about 300 Kaysersabove the D, level of Tb, as determined from their phosphorescencespectra in a rigid organic glass. In the crystalline terbium chelate Twill be somewhat lower, due to stronger van der Waals interactionsbetween the adjacent chelate molecules.

In FIG. 3 is shown a device for converting infrared signals to visiblesignals utilizing the phosphor screen of the present invention. As isshown in FIG. 3, an infrared converter 10 is comprised of screen 12having a fluorescent layer, which layer contains a phosphor system asdescribed above. In order to obtain the advantage of the low temperaturesystem as has been described this screen can be affixed to conventionalcold finger l4.

Collecting optics 18 such as parabolic mirrors are employed to collectan infrared image indicated by arrows 20 and focus said image ontoscreen 12. Screen 12 is illuminated with ultraviolet light 22 from anultraviolet source, which light is reflected by reflector 23 throughaperture 25 in mirror 24 onto said screen. As a result the fluorescentlayer of the screen will fluoresce in accordance with the intensity ofthe incident infrared light and generate an image in the visiblespectrum corresponding to the infrared image focused on the screen 12.The elliptical mirror 24 functions to focus the resulting visual imagegenerated by the screen 12 onto image orthicon tube By the mechanismpreviously discussed a visible image is produced by the infrared heatingofthe fluorescent layer when this layer is irradiated uniformally withultraviolet radiation. The image orthicon tube generates a video signalrepresenting the visual image focused thereon, which signal is amplifiedby amplifier 32 and displayed on screen 34 of cathode-ray tube 36. Inthe absence of any infrared light, the screen 12 will have a backgroundluminescence, which is added to the visual image produced when aninfrared image is focused on the screen 12. The portion of the videosignal which results from this background luminescence is cancelled outby a bias signal generated by bias signal source 38.

Instead of displaying the image with a video system as shown in FIG. 3,the red luminescent image'produced by the screen 12 can be photographedwith ordinary panchromatic film through a filter that cuts out the greenluminescent background.

Screen 12 can be constructed by depositing a thin layer 1 micrometer) ofthe phosphor on a thin layer (1 or 2 p.) of an absorber with a low molarheat capacity at cyrogenic temperatures. Graphite has been successfullyemployed as an absorber for screen 12. The temperature of cold finger 14is preferably kept below 10 K. and optimally is kept at about 4.2 K. orlower. The thermal image on the graphite from the infrared emitting orreflecting object will thus be transferred to the phosphor layer, whereit will become a visible image under ultraviolet irradiation.

In an alternate embodiment of the invention shown in FIG. 4, a flyingspot scanner system is employed to display the visual image. The systemis comprised of fluorescent screen 40, which screen is similar to thatdescribed in reference to FIG. 3. Fluorescent screen 40 is applied on aconventional cold finger 42. Screen 40 is scanned by a flyingultraviolet spot from flying spot scanner 41. The incoming infrared rays43 are imaged onto said screen by collecting optic 46 similar to thecollecting optic shown in FIG. 3. The resulting fluorescence which is afunction of the temperature rise at the scanned spot produced by theinfrared image is collected and focused on photomultiplier 48 fordetection. The resulting video signal generated by photomultiplier 48 isdisplayed on cathode-ray tube 50 which is scanned in synchronizationwith the flying spot scanner as a result of receiving horizontal andvertical sync signals from a source 51 which also supplies these syncsignals to the flying spot scanner. This system also includes biassignal source 52 which biases out the portion of the signal due to thebackground luminescence of the screen 12.

For the purpose of the calculations described below it is assumed thatthe process of infrared exposure and detection of the resulting visibleimage is shorter than the thermal relaxation time of the system.

The approximate radiant energy needed to produce a processed image canbe calculated according to eq. (5) in a system using a 2 pm. thick layerof graphite with C], l.2Xl0 cal. mole degf' (at 42 K.) and a specificgravity of 2.2. The result is This is approximately equal to 8.9 l0"joules cm. for an increase in I of 1 percent, and 3.7 l0 joules cm. fora 5 percent increase. The lowest percent increase needed to extract animage from the background luminescence is determined by the magnitude ofthe fluctuations in the number of photoelectrons produced in thephotocathode of the electronic image processing device per pictureelement.

As an example, a system similar to the one shown inFIG. 3 which receivesimages produced by a C0 laser, at a wavelength )\=l0.6 microns canillustrate the theory of the present invention. If the f number of thesystem is 1.5, the optical resolution is:

d=l .22 hf =l9.4 micrometers Since the area per resolvable square isthus 3.'76 l0 cm. the energy required per resolvable square is:

This corresponds to 3.5Xl0 photons at the wavelength of 10.6 micrometersfor a 5 percent increase and to 8.4 l0- photons for a l percentincrease.

The energy absorbed from the ultraviolet source can be of the same orderof magnitude or even greater if it is constant within a few percent andhomogeneous over the detector area. For instance, if eq. (3) isapproximately obeyed in a thermal interval AT=0.5 K. and one-halfoftheabsorbed energy is dissipated as heat, it is possible to use up to aboutSXIO ultraviolet photons of 2.6Xl0" Kaysers per photon ()\=380nanometers) per resolvable point if one starts at the lower end of thetemperature interval.

If, for instance, the total quantum yield of fluorescence in thephosphor is 0.8, equally distributed between the A and B centers, thetotal number of photoelectrons produced from B at the photocathode perresolvable point in the fluorescent screen will be 2X10 043, where a isthe fraction of the photons from B that reach the photocathode, and I3is the quantum efficiency of photoelectron production.

If a and B are 0.2 and 0.1 respectively, the fluctuations in the numberof photoelectrons per second will be:

The needed percent increase of I will then be smaller the shorter theexposure time for a given amount of absorbed energy. If 400photoelectrons are ejected in 1 second, the fluctuations will be 5percent, so that the infrared energy required per image element isgreater than l.4 l0' joules. if the same number of photoelectrons isejected in seconds the fluctuations will amount to one-half of 1percent. Then a ratio I /I,=1 .01 is sufficient, with a sensitivity of3.3)(10 joules per image element.

It is possible that only a fraction of the energy migrating away fromthe A centers reaches the emissive level of the B centers, the remainingexcitation being trapped at impurities or other defect centers, orotherwise dissipated thermally. [n such a case it may be convenient toobtain the image from the quenching of the fluorescence from the Acenters. If such quenching occurs through the energy level X eq. (1) canbe rewritten as:

, M m ear an) P 7,7)

ie n ep k ,T, :r2

where is the intensity of the luminescence of the A centers in theabsence of quenching.

While various embodiments of the invention have been described, in allinstances the fluorescent screen must meet certain criteria. Thematerial which comprises the phosphor must have a visible luminescencewith a high luminescence efficiency 0.1) when populated with ultravioletradiation or short wavelength visible light. The requirement for visibleluminescence can be met either the A center, the B center or bothcenters. A second criteria is that AE be about an order of magnitudegreater than KT with T between 2 K. and 10 k. (K=Kelvin). A thirdcriteria is that the rate K of energy migration away frorn'theluminescent A center is not more than an order of magnitude differentfrom the radiative lifetime of the A center. A fourth criteria exists ifthe A centers are nonluminescent, then the rate K of energy migration tothe luminescent B centers should not be more than an order of magnitudedifferent from the lifetime of the A centers in the absence of the Bcenters.

By providing a phosphor with the above-defined characteristics theconversion of light in the infrared portion of the spectrum into visiblelight is obtained.

I claim:

1. An infrared image converter comprising a screen having appliedthereon a phosphor, said phosphor being sensitive when irradiated withshort wavelength light in a predetermined spectrum, said phosphor beingcomprised of a plurality of centers of two different types identified asA centers and B centers, said phosphor fluorescing in accordance withthe rate of energy transfer from said A centers to said B centers whensaid A centers are populated, the rate of energy transfer from said Acenters to said B centers depending upon the temperature of saidphosphor, energy transfer from said A centers to said B centersoccurring via an energy level X which level X has an energy level higherthan that of said A centers by a value of AE wherein AB is approximatelyan order of magnitude greater than H, 7 being the temperature of thescreen and It being Boltzmanns constant, said A centers becomingpopulated when irradiated by short wavelengths light in said 4. Aninfrared image converter as recited in claim 3 wherein said means todisplay the resulting image comprises means to generate video signalsrepresenting said image and video display means responsive to said videosignals to display the image represented by said video signals.

5. An infrared image converter as recited in claim 4 wherein said meansto generate video signals comprises an image orthicon tube and means tofocus said visible image generated by said screen on said image orthicontube.

6. An infrared image converter as recited in claim 4 wherein said meansto irradiate said screen with short wavelength light comprises means toscan said screen with a spot of short wavelength light and wherein saidmeans to generate a video signal comprises a light-sensitive meanspositioned to detect the visible light generated by said screen as it isscanned by said spot of short wavelength light.

7. A device for converting light in the infrared portion of the spectruminto visible light comprising a conversion screen containing a phosphorwherein said phosphor is comprised of a plurality of different centersof two different types identified as A centers and B centers, whichfluoresce at two different wavelengths, A, and A characterized in thatat least one of said centers has a v'isibleluminescence with aluminescence efficiency 0.l when populated, said A center being capableof being populated by ultraviolet radiation or short wavelength light,energy transfers from said A center to said B center occuring via anenergy level X which level X has an energy higher than that of said Acenter by a value of AE wherein AB is approximately an order ofmagnitude greater than kT where T is the temperature of the screen and kis the Boltzmann s constant.

8. The device as set forth in claim 7 wherein the A center isluminescent.

9. The device as set forth in claim 7 wherein both the A center and theB center are luminescent.

10. The device as set forth in claim 7 wherein the A center isnonluminescent and where the rate K of energy migration to theluminescent B center is not more than an order of magnitude differentfrom the lifetime of the A center in the absence of the B center.

11. The device as set forth in claim 7 wherein said phosphor is acrystalline Tb chelate doped with Eu ions.

12. The device as set forth in claim 11 wherein said chelate has ligandswith a triplet level T within approximately 300 Kaysers above the Dlevel of Tb as determined from the phosphorescence spectra in organicglass.

13. A phosphor consisting essentially of crystalline terbium tetrakis[1,3 bis (p-methoxyphenyl) 1,3 propanedione] piperidine doped with thecorresponding europium chelate.

14. A device for converting light in the infrared portion of thespectrum into visible light comprising a conversion screen containing aphosphor comprised of a plurality of centers of two different typesidentified as A centers and B centers, characterized in that at leastone type of said centers has a visible luminescence with a luminescenceefficiency greater than 0.1 when populated, said A centers being capableof being populated by short wavelength type light, and with an energytransfer from said A centers to said B centers which occurs via anenergy level X which level X is an energy higher than that of said Acenters by a value of AE wherein AB is approximately an order ofmagnitude greater than H, where '1 is the temperature of the screen indegrees Kelvin, said temperature being not higher than 10 K., and k isthe Boltzmann constant, with said energy transfer occurring with a rateK which is not more than an order of magnitude different from thelifetime of the A centers in the absence of the B centers, and obeyingthe relation K=C exp (AE/kT) where C is a constant for a given screen.

15. The device set forth in claim 14 wherein the A centers areluminescent, and wherein the incident infrared radiation quenches theluminescence of said A centers by an amount equal to the difference I Iwhere 1,, and I are the luminescence intensities at the absolutetemperatures T and T, respectively, said intensities obeying therelation said layer of infrared absorbing material.

17. The device set forth in claim 14 comprising a means for excitationof the conversion screen with short-wavelengthtype visible light, and ameans for converting the signal resulting from the infrared heating ofthe screen into a video signal.

18. A device for converting light in the infrared portion of thespectr'um into visible light comprising a conversion screen containing aphosphor wherein said phosphor is comprised of a plurality of differentcenters of two different types identified as A centers and B centers,which fluoresce at two different wavelengths, A, and A characterized inthat at least one of said centers has a visible luminescence with aluminescence efficiency 0.1 when populated, said A center being capableof being populated by ultraviolet radiation or short wavelength light,energy transfers from said A center to said B center occurring via anenergy level x which level x has an energy higher than that of said Acenter by a value of AE wherein AE is approximately an order ofmagnitude greater than kT where T is the temperature of the screen and kis the Boltzmann s constant wherein said phosphor is crystalline terbiumtetrakis [1,3 bis (p-methoxyphenyl) 1,3 propanedione] piperidine dopedwith the corresponding europium chelate.

19. The device as set forth in claim 18 wherein said screen contains alayer of graphite.

i l= i

1. An infrared image converter comprising a screen having appliedthereon a phosphor, said phosphor being sensitive when irradiated withshort wavelength light in a predetermined spectrum, said phosphor beingcomprised of a plurality of centers of two different types identified asA centers and B centers, said phosphor fluorescing in accordance withthe rate of energy transfer from said A centers to said B centers whensaid A centers are populated, the rate of energy transfer from said Acenters to said B centers depending upon the temperature of saidphosphor, energy transfer from said A centers to said B centersoccurring via an energy level X which level X has an energy level higherthan that of said A centers by a value of Delta E wherein Delta E isapproximately an order of magnitude greater than kT, T being thetemperature of the screen and k being Boltzmann''s constant, said Acenters becoming populated when irradiated by short wavelengths light insaid predetermined spectrum, means to focus an infrared image on saidscreen, and means for irradiating said screen with short wavelengthslight in said predetermined spectrum.
 2. An infrared image converter asrecited in claim 1 wherein there is provided means to maintain saidphosphor screen at a temperature of 2* K. and higher.
 3. An infraredimage converter as recited in claim 1 wherein one of said wavelengths isin the visible spectrum whereby said screen generates a visible imageand wherein said converter further comprises means to display theresulting image.
 4. An infrared image converter as recited in claim 3wherein said means to display the resulting image comprises means togenerate video signals representing said image and video display meansresponsive to said video signals to display the image represented bysaid video signals.
 5. An infrared image converter as recited in claim 4wherein said means to generate video signals comprises an image orthicontube and means to focus said visible image generated by said screen onsaid image orthicon tube.
 6. An infrared image converter as recited inclaim 4 wherein said means to irradiate said screen with shortwavelength light comprises means to scan said screen with a spot ofshort wavelength light and wherein said means to generate a video signalcomprises a light-sensitive means positioned to detect the visible lightgenerated by said screen as it is scanned by said spot of shortwavelength light.
 7. A device for converting light in the Infraredportion of the spectrum into visible light comprising a conversionscreen containing a phosphor wherein said phosphor is comprised of aplurality of different centers of two different types identified as Acenters and B centers, which fluoresce at two different wavelengths,lambda 1 and lambda 2, characterized in that at least one of saidcenters has a visible luminescence with a luminescence efficiency >0.1when populated, said A center being capable of being populated byultraviolet radiation or short wavelength light, energy transfers fromsaid A center to said B center occuring via an energy level X whichlevel X has an energy higher than that of said A center by a value ofDelta E wherein Delta E is approximately an order of magnitude greaterthan kT where T is the temperature of the screen and k is theBoltzmann''s constant.
 8. The device as set forth in claim 7 wherein theA center is luminescent.
 9. The device as set forth in claim 7 whereinboth the A center and the B center are luminescent.
 10. The device asset forth in claim 7 wherein the A center is nonluminescent and wherethe rate K of energy migration to the luminescent B center is not morethan an order of magnitude different from the lifetime of the A centerin the absence of the B center.
 11. The device as set forth in claim 7wherein said phosphor is a crystalline Tb3 chelate doped with Eu3 ions.12. The device as set forth in claim 11 wherein said chelate has ligandswith a triplet level T1 within approximately 300 Kaysers above the 5D4level of Tb3 as determined from the phosphorescence spectra in organicglass.
 13. A phosphor consisting essentially of crystalline terbiumtetrakis (1,3 bis (p-methoxyphenyl) 1,3 propanedione) piperidine dopedwith the corresponding europium chelate.
 14. A device for convertinglight in the infrared portion of the spectrum into visible lightcomprising a conversion screen containing a phosphor comprised of aplurality of centers of two different types identified as A centers andB centers, characterized in that at least one type of said centers has avisible luminescence with a luminescence efficiency greater than 0.1when populated, said A centers being capable of being populated by shortwavelength type light, and with an energy transfer from said A centersto said B centers which occurs via an energy level X which level X is anenergy higher than that of said A centers by a value of Delta E whereinDelta E is approximately an order of magnitude greater than kT, where Tis the temperature of the screen in degrees Kelvin, said temperaturebeing not higher than 10* K., and k is the Boltzmann constant, with saidenergy transfer occurring with a rate K which is not more than an orderof magnitude different from the lifetime of the A centers in the absenceof the B centers, and obeying the relation K C exp (- Delta E/kT) whereC is a constant for a given screen.
 15. The device set forth in claim 14wherein the A centers are luminescent, and wherein the incident infraredradiation quenches the luminescence of said A centers by an amount equalto the difference IA -IA , where IA and IA are the luminescenceintensities at the absolute temperatures T2 and T1 respectively, saidintensities obeying the relation where IA is the intensity of theluminescence of the A centers in the absence of quenching.
 16. Thedevice set forth in claim 14 comprising a conversion screen consistingof: A. a layer of infrared absorbing material, and B. a layer oftemperature sensitive phosphor deposited on said layer of infraredabsorbing material.
 17. The device set forth in claim 14 comprising ameans for excitation of the conversion screen with short-wavelength-typevisible light, and a means for converting the signal resulting from theinfrared heating of the screen into a video signal.
 18. A device forconverting light in the infrared portion of the spectrum into visiblelight comprising a conversion screen containing a phosphor wherein saidphosphor is comprised of a plurality of different centers of twodifferent types identified as A centers and B centers, which fluoresceat two different wavelengths, lambda 1 and lambda 2, characterized inthat at least one of said centers has a visible luminescence with aluminescence efficiency > 0.1 when populated, said A center beingcapable of being populated by ultraviolet radiation or short wavelengthlight, energy transfers from said A center to said B center occurringvia an energy level x which level x has an energy higher than that ofsaid A center by a value of Delta E wherein Delta E is approximately anorder of magnitude greater than kT where T is the temperature of thescreen and k is the Boltzmann''s constant wherein said phosphor iscrystalline terbium tetrakis (1,3 bis (p-methoxyphenyl) 1,3propanedione) piperidine doped with the corresponding europium chelate.19. The device as set forth in claim 18 wherein said screen contains alayer of graphite.